The endocrine control of the maturation of the small intestinal epithelium will continue to be investigated in rats hypophysectomized, thyroidectomized, or adrenalectomized in neonatal life. In one series of studies, EM autoradiography is being used to examine the uptake and distribution of tritiated hexoses into the Golgi membranes and subsequently into the brush border at different ages and under different conditions of endocrine deficiency and hormone replacement. In correlation with this work, the appearance and distribution of a specific membrane component, sucrase, will be followed by use of peroxidase-labelled anti-sucrase. In another series of studies, influence of endocrine conditions on the cell cycle and rate of cell transit in suckling rats is being investigated. In a third series, work is going forward on the changing enzymatic and non-enzymatic protein composition of the brush border membrane during development, and on the influence of hormones on differentiation of the membrane. BIBLIOGRAPHIC REFERENCES: Yeh, K.Y. and Moog, F. 1975 Development of the small intestine in the hypophysectomized rat. II. Influence of cortisone, thyroxine, growth hormone, and prolactin. Develop. Biol. 47: 173-184. (5 reprints submitted 3/09/76). Yeh, K.Y. and Moog, F. 1976 Influence of thyroidectomy, adrenalectomy, and administration of thyroxine or cortisone on the growth of the intestine of the suckling rat, and on the development of intestinal alkaline phosphatase and disaccharidase activities. Submitted to the Journal of Endocrinology, 4/12/75.